1. Field of the Invention
The present invention relates to a fixing device which fixes an unfixed image onto a sheet, and an image forming apparatus including it.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or laser printer, a film heating scheme is known as a scheme of fixing an unfixed toner image onto a sheet. Japanese Patent Laid-Open No. 5-35137 proposes a method of arranging a plurality of heating resistors in a zigzag pattern on a fixing film, and heating the fixing film. Japanese Patent Laid-Open No. 2002-123113 proposes a fixing device which supplies electric power to a heating element in a non-contact manner. In the fixing device of Japanese Patent Laid-Open No. 2002-123113, electric power is supplied from a power source connected to a primary coil to a heating element coupled to a secondary coil by magnetic coupling between the primary and secondary coils each including a core and coil.
Two electrodes are necessary to supply a current to the heating element. In a fixing device of Japanese Patent Laid-Open No. 5-35137, a return path for connecting an electrode farther from a power supply part out of the electrodes at the two ends of the zigzag pattern is arranged in the same layer as that of the zigzag pattern in order to supply a current from one end side of the fixing film.
However, in Japanese Patent Laid-Open No. 5-35137, since the heating resistors are formed from the zigzag pattern, pressure unevenness is generated in the fixing film. The return path is linear and extends in a direction perpendicular to the rotational direction of the fixing film, and this also causes pressure unevenness of the fixing film. Further, the pressure unevenness sometimes produces density unevenness or a pattern in a printed image. In Japanese Patent Laid-Open No. 2002-123113, a surface exfoliate layer and elastic layer are formed on the outer surface of the cored bar of a fixing roller, and a heating layer and insulation layer are formed integrally with the inner surface of the cored bar. The presence of the cored bar in the fixing roller makes it difficult to generate pressure unevenness. However, Japanese Patent Laid-Open No. 2002-123113 does not clearly describe how to supply a current to an electrode farther from the power supply part out of the two electrodes of the heating element, in order to supply electric power to the heating element by arranging the primary and secondary coils on one end side of the fixing roller. It is technically difficult to wire a cable from one end side to the other end side of the fixing roller, as illustrated in some drawings. This is because the cable is tangled around a stay which pushes a heating film against a pressing roller from the inner side of the heating film.